The present disclosure relates to a separator for a secondary battery and a method of preparing the same, and more particularly, to a separator for a secondary battery having a hydrophobically-treated surface and a method of preparing the same.
Lithium ion secondary batteries commonly include anodes having composite lithium oxides, cathodes having materials capable of the absorption and emission of lithium ions, as well as separators and non-aqueous electrolyte solutions interposed between the anodes and the cathodes. Such lithium ion secondary batteries are formed of electrodes having such anodes, cathodes, and separators interposed therebetween, and stacked on each other, or electrodes having a columnar shape, and wound after the stacking of the electrodes.
The separators function to electrically insulate the anodes from the cathodes, and to support the non-aqueous electrolyte solutions. Porous polyolefin is commonly used to form the separators of such lithium ion secondary batteries. Porous polyolefin has excellent electrical insulating properties and ion transmitting properties, and is widely used in the separators of lithium ion secondary batteries or those of condensers.
Since lithium ion secondary batteries have a high output density and a high capacity density, but include non-aqueous electrolyte solutions, for example, organic solvents, such non-aqueous electrolyte solutions are decomposed by heat generated in an abnormal state, such as a short circuit or overcharging, and in worst case scenarios, may ignite. In order to address these issues, lithium ion secondary batteries have several safety functions, one such safety function being a separator shutdown function.
The separator shutdown function is provided such that the pores of separators are occluded by thermally molten resin materials when lithium ion secondary batteries generate abnormal heat, so as to suppress ionic conduction in the non-aqueous electrolyte solutions, thereby stopping the progress of electrochemical reactions.
In general, it is known that as a shutdown temperature is low, the safety of lithium ion secondary batteries is high. One reason why polyethylene is used as a components of separators is that polyethylene has a moderate shutdown temperature. As such separators, for example, uni-axially or bi-axially stretched resin films are used to porosify the separators and increase the strength thereof.
However, when separators are shut down, the membranes of the separators themselves can be shrunk, and anodes and cathodes may come into contact with each other to cause a secondary problem, such as an internal short circuit. Thus, there is a need for improving the safety of lithium ion secondary batteries by reducing the thermal contraction of separators by increasing the heat resistance thereof.
In order to solve problems, such as an internal short circuit and the like caused by such shrunk membranes, stacking layers, for example, porous layers, containing non-conductive particles such as inorganic particles on polyolefin-based organic separators or on electrodes, for example, positive or negative electrodes, has been proposed. Also, disclosed is a separator having an inorganic oxide layer deposited on the surface of a porous polyolefin-based substrate and on the internal surfaces of pores thereof using atomic layer deposition (ALD) (Japanese Patent Publication No. 2012-181921).
However, when porous layers are formed on porous substrates such as polyolefin substrates, the porous layers are formed using wet processes. Since inorganic substances included in separators have hydrophilicity, it is easy for a large amount of water to be included in the pores of the separators. Such water may have a negative influence on lithium ion secondary batteries, and may result in problems such as generating gas through a reaction with electrolytes or significantly reducing the lifespan of lithium ion secondary batteries by allowing lithium ions to be used due to the water.
Thus, separators are required to maintain their water contents in order to reliably exhibit excellent lifespan properties. Also, disclosed are a technology of introducing a functional group to inorganic particles (Korean Patent Publication No. 2014-0003429), a technology of introducing a surface modifier to inorganic particles (Korean Patent No. 1223081), a technology of coating the surface of inorganic particles with a coupling agent (Korean Patent No. 1358761), and a technology of introducing a layer including polyvinylidene fluoride (PVDF) to the surface of a separator (Japanese Patent Publication No. 2014-026946), in order for the water contents of separators to remain low.